Tumor spreading occurs as a consequence of continuous cell membrane protrusion, invasion and migration, the activities that require dynamic actin cytoskeleton reorganization. Actin assembly in the cell leading edge is achieved by the functions of Arp2/3 complex, WASP/WAVE family proteins and cortactin, which are activated by Rho small GTPases and protein tyrosine kinase Src. Our recent study has uncovered that cortactin binds to the protein product of missing in metastasis (MIM) gene, expression of which is frequently lost in a subset of aggressive tumor cells and advanced human bladder cell carcinomas. Overexpression of MIM inhibits markedly the motility of tumor cells or normal cells mediated by growth factors, and restrains the lamellipodia formation induced by constitutively activated Rac1, a member of the Rho GTPase family. Within cells MIM colocalizes with and is able to coprecipitate with Rac1. In vitro, MIM inhibits the actin polymerization mediated by WASP. Interestingly, MIM undergoes a rapid tyrosine phosphorylation in concurrent with transient interaction with cortactin in response to growth factors. Phosphorylation at two tyrosine residues is required for MIM to inhibit Rac1. Based on these data, we hypothesize that MIM is implicated in a novel signaling pathway antagonizing the function of Rac in the assembly of actin cytoskeleton, the activity that is subjected to a regulation through tyrosine phosphorylation. To test this hypothesis, we propose to characterize the mechanism for MIM to antagonize Rac. Therefore, we will determine whether MIM interacts directly with Rac and examine whether the direct association is necessary and sufficient to inhibit Rac. We will also analyze the role of IRSp53 and WAVE, the effectors of Rac, in the function of MIM, and test whether inhibition of Rac is the mechanism for MIM to modulate cell motility induced by growth factors. We will delineate the signaling pathway for MIM phosphorylation and examine whether Src is the primary tyrosine kinase responsible for the PDGF mediated tyrosine phosphorylation of MIM, and analyze tyrosine phosphorylation of MIM in Src knockout cells and the cells overexpressing temperature sensitive v-Src. We will also explore the role of tyrosine phosphorylation in the regulation of MIM with respect to interaction with Rac and cortactin, and cell motility. Finally, we will examine the role of MIM expression at low levels in the motility of tumor cells by silencing MIM using RNA interference in MIM expression cells, and examine whether suppression of MIM expression will lead to increase in lamellipodia development, Rac activation, actin polymerization in cells and cell motility. It is anticipated that achievement of the goal of this proposal will direct our future effort to understand how MIM contributes to tumor progression.